In a boiler of this type, with a duct connecting the combustion hearth to a separator cyclone, particles are separated out in the separator cyclone and are recycled through the combustion hearth. Gas is evacuated via a chimney after passing through conventional heat exchangers situated downstream from the separator cyclone. Reducing nitrogen oxides into inert molecular nitrogen is a corrective measure which diminishes the amount of nitrogen oxides that are discharged with the gases exhausted by the chimney.
In general, ammonia is injected into the flow of particles and gas in order to reduce the nitrogen oxides by a reaction scheme that is known as non-catalytic selective reduction. It is nowadays accepted that the reduction reaction in the installation is influenced by three main parameters, namely: temperature, transit time, and the mixing of the reactive ammonia with the nitrogen oxides.
European patent application EP 0 690 266, published on Jan. 3, 1996, describes a boiler in which ammonia is injected via an opening formed in the wall of the top portion of the duct, the opening being disposed at a shorter distance from the combustion hearth than the separator cyclone. That method of injection is relatively simple to implement. Nevertheless, injecting through the wall of the duct does not make it possible to achieve complete mixing of the reactive ammonia and the nitrogen oxides. Although the flow of particles and gas is turbulent, it is nevertheless dominated by a speed component that is parallel to the longitudinal axis of the duct and that restricts penetration of the reactive ammonia into a layer that is in contact with the wall of the duct.